1. Field of the Invention
The invention relates to a battery system in which discharge of an alkali secondary battery is controlled on the basis of a positive electrode potential of the alkali secondary battery.
2. Description of Related Art
In International Patent Application No. 2013/105140 (W/O 2013/105140), the occurrence of a secondary reaction in a positive electrode (variation in a structure or a crystallinity of the positive electrode) is suppressed by measuring a positive electrode potential using a reference electrode, and reducing an upper limit power that can be output (discharged) from a single cell when the positive electrode potential is lower than a threshold potential. W/O 2013/105140 also describes use of a battery model method to estimate the positive electrode potential.
When a reference electrode is used, as in W/O 2013/105140, the positive electrode potential can be measured, but when the reference electrode is omitted, the positive electrode potential cannot be measured.
However, when a battery model method is used, as described in W/O 2013/105140, the positive electrode potential can be estimated without the use of a reference electrode. In an alkali secondary battery, a memory effect occurs, and the positive electrode potential of the alkali secondary battery is dependent on a state of charge (SOC) and the memory effect of the secondary battery. In other words, a voltage value and the positive electrode potential of the alkali secondary battery vary due to the memory effect even when the SOC of the alkali secondary battery remains identical.
In the battery model method described in W/O 2013/105140, the memory effect of the alkali secondary battery is not taken into account, and therefore room for improvement remains with respect to estimation of the positive electrode potential of the alkali secondary battery.